Bridge Corrosion

1
Bridge Corrosion

• A look at the corrosion of steel in Bridges

2
Steel Life Cycle
Iron ore
Blast furnace - reduction to iron
Steelmaking furnace (add carbon)
Oxidation of iron - rust
Structural steel
3
How a bridge cable is made

• Cables are made of thousands of individual steel
  wires bound tightly together. Steel, which is
  very strong under tension, is an ideal material
  for cables a single steel wire, only 0.1 inch
  thick, can support over half a ton without
  breaking.

4
Suspension Bridges

• The towers are built up from reinforced concrete
  or steel, with a gap for the main roadway. Then
  the anchorages are installed to hold the cables.
  Very thin steel cable is spun from between the
  anchorages until it can be gathered together to
  form two large cables. Other cables are then
  dropped down from the main cables and the
  sections of prefabricated roadway are attached to
  them.

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Corrosion

• This is the last stage of a steel products life
  cycle.
• When steel is exposed to water and oxygen, it
  rusts
• Forms iron oxide

6
Ways to prevent corrosion

• Protective coatings
• plating with another metal
  painting
• galvanized steel

7
Preventing Corrosion

• Use corrosion resistant metal
• ex stainless steel
• Only problem?
• Stainless steel is an alloy of chromium
• and is therefore a heck of a lot more
• expensive than regular steel!

Gateway Arch, St. Louis
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Preventing Corrosion

• Anodic/Cathodic protection
• a technique used to control the corrosion of a
  metal surface by making it work as a cathode.
  This is achieved by placing in contact with the
  metal to be protected another more easily
  corroded metal to act as the anode.

Aluminum cathodes on steel
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Anodic region

• The negative part of the cell to which current
  flows
• Oxidation occurs (gaining electrons)
• Rust forms
• The more reactive metal!

Zinc anode
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Cathodic region

• The positive part of the cell to which current
  flows from
• No rust forms
• Less reactive metal
• The area turns pink in phenolphthalein indicator
  (its basic!)

Steel (Iron) pipe cathode
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What do you think?

• If you were to use anode/cathode protection for a
  piece of copper wrapped in zinc, which metal
  would be rusted?
• (remember the more reactive metal will be the
  anode and rust!)

12
Corrosion Impact

• Approximately 20 of all the iron and steel
  produced is used to repair or replace corroded
  structures. (and this cost billions of dollars
  each year in repair and maintenance costs!)